Photovoltaic (PV) devices are known in the art (e.g., see U.S. Pat. Nos. 6,784,361, 6,288,325, 6,613,603 and 6,123,824, the disclosures of which are hereby incorporated herein by reference). Amorphous silicon (a-Si) and CdS/CdTe type photovoltaic devices, for example, each include a front contact or electrode. Typically, the front electrode is made of a transparent conductive oxide (TCO) such as tin oxide or zinc oxide formed on a substrate such as a glass substrate. Accordingly, it will be appreciated that zinc oxide doped with Al (ZnAlOx) is a known TCO material for use as an electrode for a photovoltaic device.
Unfortunately, transparent conductive ZnAlOx has a relatively low refractive index (n) of around 2.0. A TCO layer having such a low refractive index (n), if provided in certain locations of a PV device, tends to reflect significant amounts of incoming solar energy away from the active semiconductor film of the photovoltaic device. In other words, when located at certain positions, the low refractive index material causes significant amounts of incoming energy to be wasted as it is reflected away from the active layer thereby reducing the efficiency of the photovoltaic device.
It is apparent from the above that there exists a need in the art for an improved TCO material which, when positioned in certain locations, is capable of reducing reflections of incoming solar energy without blocking transmission thereof, so as to improve efficiency of the photovoltaic device.
In certain example embodiments, a transparent conductive oxide (TCO) of the front electrode for use in a photovoltaic device is of or includes titanium oxide doped with one or more of Nb, Zn and/or Al. Additional conductive layers may also be provided in the front electrode in certain example embodiments. It has been found that the use of TiOx(:Nb) or TiZnOx(:Al and/or Nb) TCO, in a front electrode of a photovoltaic device, is advantageous in that such materials have a high refractive index (n) (e.g., of at least 2.15, more preferably at least 2.2, even more preferably at least 2.3, and possibly at least 2.4 at 550 nm) and have a higher transparency to solar energy used by the PV device than conventional titanium suboxide (TiOx). Thus, the use of such materials in the context of a front electrode of a photovoltaic device (especially when positioned adjacent and contacting the active semiconductor film of the device) reduces light reflection due to the high refractive index, and increases transmission into the active semiconductor film due to the higher transmission characteristics thereof, thereby improving efficiency of the device.
In certain example embodiments of this invention, the TCO may be sputter-deposited in a non-stoichiometric oxygen deficient form, or may be deposited in any other suitable manner. Sputtering at approximately room temperature may be used for the deposition of the electrode in certain example instances, although other techniques may instead be used in certain instances.
In certain example embodiments, the electrode of or including TiOx(:Nb) or TiZnOx(:Al and/or Nb) may be used as any suitable electrode in any suitable electronic device, such as a photovoltaic device, a flat-panel display device, and/or an electro-optical device.
In certain example embodiments of this invention, the TCO (e.g., TiOx(:Nb) or TiZnOx(:Al and/or Nb)) layer or the overall front electrode may have a sheet resistance (Rs) of from about 7-50 ohms/square, more preferably from about 10-25 ohms/square, and most preferably from about 10-15 ohms/square using a reference example non-limiting thickness of from about 1,000 to 2,000 angstroms, although other thicknesses are possible.
In certain example embodiments of this invention, there is provided a photovoltaic device comprising: a front substrate; a front electrode; a semiconductor film, wherein the front electrode is located between at least the front substrate and the semiconductor film; and wherein the front electrode of the photovoltaic device comprises a first conductive layer and a second conductive layer, wherein the second conductive layer is located between at least the first conductive layer and the semiconductor film, and wherein the second conductive layer comprises titanium zinc oxide doped with aluminum and/or niobium.
In other example embodiments, there is provided a photovoltaic device comprising: a front glass substrate; a front electrode; a semiconductor film, wherein the front electrode is located between at least the front substrate and the semiconductor film; and wherein the front electrode of the photovoltaic device comprises a first conductive layer and a second conductive layer, wherein the second conductive layer is located between at least the first conductive layer and the semiconductor film, and wherein the second conductive layer comprises titanium oxide doped niobium.